Tyrannosaurus rex exemplifies every image conjured by the word "dinosaur": massive, ferocious-looking, and extinct. It is the only dinosaur that is commonly known by both its generic and specific names. Despite this, fossil evidence for this animal was surprisingly scant until quite recently, and it was only in the 1990s that important gaps in our understanding of Tyrannosaurus were filled in.

Tyrannosaurus was one of the largest of predatory dinosaurs. Some individuals measured as much as 42 feet (12.8 m) long and were up to 13 feet (4 m) tall at the hip, with a skull more than 5 feet (1.5 m) long. By any standards, Tyrannosaurus was a tremendous animal. This giant was also one of the last of the nonavian dinosaurs. All the Tyrannosaurus skeletons have come from the latest Cretaceous deposits of the United States and Canada, although some researchers regard Tarbosaurus, a large tyrannosaurid from slightly older deposits of Mongolia, to have been a form of Tyrannosaurus.

Like other tyrannosaurids, Tyrannosaurus had two very short forelimbs and only two functional fingers on each hand. The forelimb of the longest known specimen was hardly any longer than the forearm of an adult human. The front teeth were D-shaped in cross-section, and each cheek bore 12 rather robust teeth, which were shaped more like serrated bananas than the steak-knife shapes seen in most other theropods. Henry Fairfield Osborn first described Tyrannosaurus in 1905 from fossils that Barnum Brown had collected in Montana. More fossils that came to light in the course of the next few years allowed Osborn to amplify his interpretation.

A Tyrannosaurus rex tooth cast taken from a well known fossil skeleton named Stan. This well preserved tooth includes the root and measures 11 inches in total length. The serrations are very clear and the tooth is thick and round in shape, unlike the thin steak-like teeth on many other carnivores such as Velociraptor.

These toughened teeth of Tyrannosaurus rex make them ideal for crushing the large bones of its prey.

Over the years new discoveries were made, including several more complete specimens. However, no hand came to light until 1990, when John Horner, of Montana State University, published an account of a Tyrannosaurus specimen from Montana in which the hand was preserved. This discovery confirmed the presence of only two digits-something that scientists had suspected by analogy with other tyrannosaurids. Osborn's reconstructions showed a three-fingered hand-sensible guess, as all other theropods known at the time of his reconstructions had three fingers.

In 1991, a group of commercial fossil-hunters discovered "Sue" on a ranch in South Dakota. It is perhaps the largest and most complete Tyrannosaurus skeleton ever found. A legal battle over ownership followed this discovery. Finally the courts awarded the fossil to the rancher, who in 1997 sold it at auction to the Field Museum in Chicago. Researchers have high hopes for Sue and expect it will significantly extend our knowledge of Tyrannosaurus. A high-resolution computed tomographic (CT) study of the skull is giving scientists access to what were hitherto inaccessible internal details of this dinosaur's head.

The predatory habits of Tyrannosaurus are still unclear. Some people, presuming it was a slow mover and citing the smallness of its forelimbs, maintain that it must have been a scavenger. Others, who claim that it was an active hunter, point to the strength of this animal's teeth and the evidence that bite marks found on Triceratops bones seem to have been made by Tyrannosaurus teeth.